Deformable mirror method and apparatus including bimorph flexures and integrated drive

ABSTRACT

An apparatus comprising a substrate; and a platform elevated above the substrate and supported by curved flexures. The curvature of said flexures results substantially from variations in intrinsic residual stress within said flexures. In one embodiment the apparatus is a deformable mirror exhibiting low temperature-dependence, high stroke, high control resolution, large number of degrees of freedom, reduced pin count and small form-factor. Structures and methods of fabrication are disclosed that allow the elevation of mirror segments to remain substantially constant over a wide operating temperature range. Methods are also disclosed for integrating movable mirror segments with control and sense electronics to a produce small-form-factor deformable mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication Nos. 60/425,049 entitled Reduced Rotation MEMS DeformableMirror Apparatus and Method, and U.S. Provisional Patent Application No.60/425,051 entitled Deformable Mirror Method and Apparatus IncludingBimorph Flexures and Integrated Drive, both filed Nov. 8, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a methods and structures for elevatinga platform above a substrate and for producing a controlled motion ofthat platform. It also relates to MEMS deformable mirror (“DM”) arrays,and more particularly to long-stroke MEMS deformable mirror arrays foradaptive optics applications.

[0004] 2. Description of the Related Art

[0005] Adaptive optics (“AO”) refers to optical systems that adapt tocompensate for disadvantageous optical effects introduced by a mediumbetween an object and an image formed of that object. Horace W. Babcockproposed the concept of adaptive optics in 1953, in the context ofmirrors capable of being selectively deformed to correct an aberratedwavefront. See John W. Hardy, Adaptive optics for astronomicaltelescopes, Oxford series in optical and imaging sciences 16, OxfordUniversity Press, New York, 1998. Since then, deformable mirrors (DM)have been proposed for a variety of AO applications, although they haveyet to be implemented in many such proposed applications.

[0006] The general operation of a DM is shown schematically in FIG. 1,in which a DM 100 reflects an aberrated wavefront 105, resulting in adesired planar wavefront 110. The DM shape is dynamically adapted tocorrect the path-length variations of the inbound aberrated wavefront.That is, by selectively deforming the mirror to decrease or increase thepath length for specific portions of the aberrated wavefront, theaberrations in the reflected wavefront are corrected. The amount oflocal displacement needed of the DM surface is generally approximatelyequal to half the path-length variations in the aberrated wavefront. Theexact scale factor depends on the angle at which the aberrated wavefrontstrikes the deformable mirror.

[0007] A prior art AO system is shown schematically in FIG. 2. Thisexample is particularly related to an astronomical telescopeapplication, but the general principles of AO shown here areillustrative of other applications. In FIG. 2, an aberrated wavefront105 enters the optical system 205 where it is modified as it reflectsoff a DM 100. Aberrations in the wavefront reflected from the DM are theerror signal for a computer-controlled feedback loop. The reflectedwavefront 110 enters a dichroic beam splitter 220; the infraredwavelengths pass to a science camera 225 and the visible wavelengthsreflect toward a wavefront sensor 230. The wavefront sensor measures thewavefront slope at discrete points and sends these data to a wavefrontreconstructor 235. The wavefront reconstructor 235 determines theremaining wavefront aberrations in the corrected wavefront. An actuatorcontrol block 240 calculates actuator drive signals to correct theremaining wavefront errors, which are sent from the block 240 to the DM100, thus closing the feedback loop. In this way, the DM is continuouslydriven in such a way as to minimize the aberrations in the reflectedwavefront, thereby improving image resolution at the science camera.

[0008] AO systems have been proposed and demonstrated for improvingresolution in a number of imaging applications. In astronomy, forexample, AO has been used to correct aberrations introduced by motion ofthe atmosphere, allowing ground-based telescopes to exceed theresolution provided by the Hubble Space Telescope under some observingconditions. In the field of vision science, AO has been shown to offerbenefits, for example, for in-vivo retinal imaging in humans. Here, AOsystems can compensate for the aberrations introduced by the eye,improving lateral image resolution by a factor of three and axialresolution by a factor of ten in confocal imagers. This has allowedindividual cells to be resolved in living retinal tissue, a capabilitythat was not present before the advent of AO.

[0009] In addition to improving image resolution, AO systems can be usedto improve confinement of a projected optical beam traveling through anaberrating medium. Examples of applications in this category arefree-space optical communication, optical data storage and retrieval,scanning retinal display, and laser-based retinal surgery.

[0010] A number of characteristics are commonly used to compareperformance of DM designs. Fill-factor is the fraction of the DMaperture that is actively used to correct wavefront aberrations. Mirrorstroke is the amount of out-of-plane deformation that can be induced inthe DM surface. The number of degrees-of-freedom is a measure of thespatial complexity of the surface shapes the DM is capable of assumingand is related to the number of individual actuators that are used todeform the mirror surface. DM aperture diameter, DM device size, controlresolution, operating temperature range, power consumption, frequencyresponse and price are also generally considered when selecting a DM fora given application. For example, astronomical imaging typicallyrequires mirror stroke in the range of a few micrometers, frequencyresponses in the kilohertz range and aperture sizes on the order of afew centimeters to a few meters. Systems for imaging structures in thehuman eye, by contrast, generally require mirror stroke on the order of10 micrometers or greater, frequency responses in the tens to hundredsof Hertz range, and aperture sizes on the order of one centimeter orless.

[0011] Despite the advantages outlined above, AO has not beenuniversally adopted, even in the aforementioned applications. Twoimportant factors that have impeded the widespread adoption of AO arethe high cost and limited stroke of available DMs.

[0012] DM designs can be broadly divided into two classes;continuous-face-sheet designs and segmented designs.Continuous-face-sheet DMs have a reflective surface that is continuousover their whole aperture. The surface is deformed using actuators,typically mounted behind it, that push or pull on it to achieve adesired deformation. This type of DM has been implemented, for example,by mounting an array of piezoelectric actuators to the rear surface of asomewhat flexible glass or ceramic mirror. Because the optical surfaceis continuous and rather inelastic, large actuation forces are requiredto deform the mirror, and the resulting mirror stroke is small,typically less than 5 micrometers. The continuous surface also meansthat the deformation produced by each actuator is not tightly confinedto the area of the mirror directly connected to it, but instead mayextend across the whole mirror aperture, making precise control of theoverall mirror deformation problematic. Because of the way they areconstructed, such DMs are also comparatively large, having apertures onthe order of 50 mm or greater. This large size precludes theirdeployment in many optical systems that might otherwise benefit from AO.Their fabrication methods also make these DMs expensive to manufactureand do not permit easy integration of control electronics into the DMstructure.

[0013] A number of continuous-face-sheet DMs using microfabricationtechniques that offer the potential to reduce DM size and cost have beencreated. Vdovin and Sarro, in “Flexible mirror micromahined in silicon”,Applied Optics, vol. 34, no. 16 (1995), disclose a DM fabricated byassembling a metalcoated silicon nitride membrane above an array ofelectrodes that are used to deform the membrane by electrostaticattraction.

[0014] Bifano et al. disclose an alternative microfabricatedcontinuous-face-sheet DM in “Microelectromechanical Deformable Mirrors”,IEEE Journal of Selected Topics in Quantum Electronics, vol. 5 no. 1(1999). Their design relies on the removal of a sacrificial layer tocreate cavities underneath the mirror surface that define the maximumtravel range of each mirror actuator. U.S. Pat. No. 6,384,952 to Clarket al. (2002) discloses a continuous-face-sheet DM that employs amirrored membrane fabricated, for example, from metalcoated siliconnitride and actuated by an array of vertical comb actuators disposedunderneath the membrane. Use of vertical comb actuators can providehigher force for a given applied voltage than the parallel plateelectrostatic actuators used in other continuous-face-sheet designs.

[0015] In contrast to the continuous-face-sheet designs discussed above,segmented DM designs divide the DM aperture into a number of generallyplanar mirror segments, the angle and height of each segment beingcontrolled by a number of actuators. Segmented designs are advantageousin that they allow the area of influence of each actuator to be tightlyconfined, simplifying the problem of driving the mirror to a particulardesired deformation. Segmenting the mirror surface also eliminates theneed to deform a comparatively inflexible optical reflector to produce adesired DM surface shape. Rather, the individual mirror segments aretilted, raised and lowered to form a piecewise approximation of whateverdeformation is required to correct the aberrations of the incomingwavefront. Segmenting the surface can therefore result in a lower forcerequirement for a given surface deformation, enabling the high-strokeDMs that are needed for many AO applications.

[0016] A number of inventors have disclosed segmented DM designs thatmay be constructed using microfabrication techniques. U.S. Pat. No.6,175,443 to Aksyuk et al. (2001) discloses an array of conductivemirror elements, connected together by linking members that act assupports, suspending the mirror array above an actuating electrode.These linking members also serve to keep the mirror array in anapproximately planar configuration when no actuating voltage is applied.Energizing the electrode results in an attractive force between it andthe mirror segments, deforming the array into a curved configuration.

[0017] U.S. Pat. No. 6,028,689 to Michalicek et al. (2000) discloses anarray of mirror segments attached to a substrate by posts, each segmentcapable of tilting about two axes and also moving vertically,perpendicular to the array, under the influence of applied controlvoltages.

[0018] U.S. Pat. No. 6,545,385 to Miller et al. (2003) discloses methodsfor elevating a mirror segment above a substrate by supporting it onflexible members that can bend up out of the substrate plane. Thisprovides a large cavity underneath the mirror segment, not limited bythe thickness of the sacrificial materials used in its fabrication, andoffering the potential for large mirror stroke.

[0019] Helmbrecht, in “Micrmirror Arrays for Adaptive Optics”, PhD.Thesis, University of California, Berkeley (2002), discloses a segmentedDM for use in AO applications, that exhibits high fill-factor, highmirror quality and offers the potential for high mirror stroke.

SUMMARY OF THE INVENTION

[0020] It is therefore an object of the present invention to provideimproved methods and structures for elevating a platform above asubstrate and for precisely controlling the tip, tilt and piston motionof that platform.

[0021] A further object of the invention is to provide ahigh-degree-of-freedom DM which can be used to compensate for largeoptical wavefront aberrations, without the need for temperature controlor monitoring.

[0022] Another object of the invention is to provide ahigh-degree-of-freedom, high-stroke DM with integrated controlelectronics in a small form-factor configuration.

[0023] A further object of the invention is to provide ahigh-degree-of-freedom, high-stroke DM with integrated sense electronicsin a small form-factor configuration.

[0024] Yet another object is to provide a high-degree-of-freedom DM witha greatly reduced control-pin count.

[0025] A further object of the invention is to provide asmall-form-factor DM that can be used in clinical ophthalmic instrumentsto correct wavefront aberrations of the human eye.

[0026] A further object of the invention is to provide ahigh-degree-of-freedom, high-stroke DM that can be fabricated at lowcost.

[0027] A further object of the invention is to provide atemperature-insensitive, high-fill-factor, segmented piston-tip-tilt DM,having segments with improved optical flatness.

[0028] Yet another object of the invention is to provide ahighly-reliable DM, capable of operating over many millions of actuationcycles.

[0029] A further object of the invention is to provide ahigh-degree-of-freedom DM comprising actuators that may be operatedlargely independently, in order to provide correction for differentareas of an optical wavefront.

[0030] A further object of the invention is to provide a DM that can bebatch fabricated using IC-compatible fabrication methods and materials.

[0031] A further object of the invention is to provide ahigh-degree-of-freedom DM with reduced power consumption.

[0032] In accordance with the above objects, the invention, roughlydescribed comprises an apparatus including a substrate and a platformelevated above the substrate and supported by curved flexures, whereinthe curvature of said flexures results substantially from variations inintrinsic residual stress within said flexures.

[0033] In another embodiment, the invention comprises a tiled array ofmirror segments, each supported by a number of curved flexures attached,at one end, to the underside of the segment and, at the other end, to asubstrate. A number of independently addressable actuators are used toapply forces to each mirror segment, causing it to move in a controlledmanner. The application points of the actuating forces and the locationsof the support flexures are placed so as to allow each segment to betilted about two distinct axes substantially parallel to the substrateand translated along an axis substantially perpendicular to thesubstrate. The invention may optionally include electronic circuitsembedded in the substrate for the purpose of addressing the individualactuators and/or sensing the state of a given mirror segment. Theinvention includes methods and structures for improved flexures forsupporting and elevating the segments above the substrate. Moreparticularly, the invention provides methods and apparatus forfabricating mirror segments supported by curved flexures, the curvatureof which is induced, principally or entirely, by variations in intrinsicresidual stress through the thickness of the flexure material ormaterials. The invention also includes methods for separatelyfabricating the MEMS portion of the inventive apparatus and theelectronics portion, and then integrating the two to form the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1: Illustration of prior art use of a deformable mirror tocorrect an aberrated wavefront.

[0035]FIG. 2: Illustration of prior art adaptive optic (AO) system.

[0036]FIG. 3A: Partial cutaway perspective view of a first embodiment ofthe invention.

[0037]FIG. 3B: Perspective view of the improved flexure according to afirst embodiment of the invention.

[0038]FIG. 4A: Flow diagram of the process steps required to fabricate afirst embodiment of the invention.

[0039]FIG. 4B: Schematic cross-sections through structures fabricated atvarious process steps in a first embodiment of the invention.

[0040]FIG. 5: Schematic cross-section through the MEMS structures for asingle mirror segment in a first embodiment of the invention.

[0041]FIG. 6: Schematic cross-section through the portion of the CMOSsubstrate underlying a single mirror segment in a first embodiment ofthe invention.

[0042]FIG. 7A: Schematic cross-section through a single mirror segmentand underlying structures before MEMS release and passivation layerremoval in a first embodiment of the invention.

[0043]FIG. 7B: Schematic cross-section through a single mirror segmentand underlying structures after MEMS release and passivation layerremoval, in a first embodiment of the invention.

[0044]FIG. 8: Table of coefficients of thermal expansion for severalcandidate materials for construction of curved flexures.

[0045]FIG. 9: Partially exploded perspective view of a third embodimentof the invention.

DETAILED DESCRIPTION

[0046] Methods and structures for elevating one or more platforms abovea substrate and for controlling the tip, tilt and piston motion of thoseplatforms with high precision are hereinafter described. Severalembodiments are described in which a plurality of such platforms aretiled to form a large-stroke segmented piston-tip-tilt deformable mirror

[0047]FIG. 3A shows a partial cutaway perspective view of a firstembodiment of a DM incorporating the improved methods and structures.The DM is formed on a substrate 300, which may be a silicon wafer orchip containing embedded addressing and sensing circuits (not shown). Ontop of the substrate 300 are formed a number of control electrodes 370that are electrically isolated from one another and electricallyconnected to the embedded addressing and sensing circuits. In the firstembodiment, the control electrodes 370 are arranged in groups of threeand are rhombic in shape, so that the footprint of each group isessentially hexagonal. Disposed around each group of three controlelectrodes 370, are three conductive ground pads 310, fabricated fromthe same material as the control electrodes 370. The ground pads 310 areelectrically isolated from the control electrodes 370 and electricallyconnected to a ground plane or to circuits embedded in the substrate300. Attached to one end of each ground pad 310 is a first anchorportion 350 of a flexure 320. The flexure, in the first embodimentcomprises two layers, a first flexure layer 330 formed from conductivepolycrystalline silicon and a second flexure layer 340 formed fromsilicon nitride (SixNy). The first anchor portion 350 is bothmechanically and electrically connected to the ground pad 310 so thatthe conductive first flexure layer 330 is held at the same electricalpotential as the ground pad 310. The second flexure layer 340 is rigidlyattached to the underside of the first flexure layer 330 and extendsover a portion of the length of the flexure 320. The purpose of thesecond flexure layer is to provide a residual stress difference betweenthe top and bottom portions of the flexure 320, causing the flexure 320to bend up out of the plane of the substrate 300.

[0048] The end of the flexure 320 opposite the first anchor portion 350terminates in a second anchor portion 360. FIG. 3B is a detailperspective view of one flexure 320, showing the first anchor portion350, the second anchor portion 360, the first flexure layer 330, thesecond flexure layer 340, and the ground pad 310 underlying the flexure.

[0049] Referring again to FIG. 3A, the second anchor portion 360 ismechanically and electrically connected to the underside of a mirrorsegment 380. The mirror segment is any one individual mirror of the DMdevice. Thus the mirror segment 380 is held at some elevation above thesubstrate 300. In the first embodiment, this elevation is on the orderof 50 micrometers. The mirror segment is electrically conductive andtherefore is held at the same potential as the ground pad 310. In thefirst embodiment, the mirror segment 380 is hexagonal in shape and isformed from a 20 micrometer-thick layer of single crystal silicon and iscoated on its top surface with an optical coating, which may be a highlyreflective metal layer. The mirror segment diameter in the firstembodiment is on the order of 500 micrometers.

[0050] For the sake of clarity, FIG. 3A shows only three mirror segments380. However, an exemplary embodiment of the DM comprises an array of121 nominally identical elevated mirror segments 380 disposed over thesubstrate so as to form a larger, segmented mirror surface,approximately circular in outline and having inter-segment gaps of 5micrometers.

[0051] The following is a general overview of the process of the currentinvention for fabricating the first embodiment of the DM. The processinvolves separately fabricating the MEMS structure and the addressingand sensing circuits on two separate wafers, then assembling themtogether as shown in FIGS. 4A and 4B. FIG. 4A is a process flow diagramand FIG. 4B illustrates the corresponding structure at each step. Asshown at step 400, each mirror segment 380 is fabricated by reactive ionetching (RIE) the top single crystal silicon “device” region of a bondedsilicon-on-insulator wafer (BSOI). At step 405, the wafer is then coatedwith a sacrificial layer to fill the trenches left by the previous etch,provide a temporary support for various mechanical structures of the DM,and optionally to act as a dopant source for undoped polysiliconregions. This sacrificial layer might typically be phosphorus-dopedsilicate glass deposited by low pressure chemical vapor deposition(LPCVD). Alternatively, in cases where the sacrificial layer is notrequired to act as a dopant source, silicon oxide deposited by atetraethoxysilane (TEOS) process might be used.

[0052] As shown at step 410, the PSG region is next patterned to definethe attachment points for the second anchor portions 360 of theflexures; in some instances the patterning may include an etching step.At step 415, a one micron undoped amorphous polysilicon layer and a PSGlayer are deposited by LPCVD and annealed at 950° C. for six hours todope and tune the residual stress of the polysilicon layer toapproximately −40 MPa, where the negative sign denotes compressivestress. The top PSG layer is then removed at step 420 using a wethydrofluoric (HF) acid etch and the polysilicon layer is patterned andetched to define the first flexure layer 330 at step 425. Siliconnitride (SixNy) is then deposited by LPCVD at step 430, and patternedand etched to define the second flexure layer 340 at step 435. At step437, conductive metal pads are deposited, for example by electroplating,on to the first anchor portion 350 of the flexures. These metal padswill serve as the electrical and mechanical attachment points betweenthe flexures and the substrate 300.

[0053]FIG. 5 schematically illustrates a cross-section through the MEMSstructure 500 supporting a single mirror segment, completed up to thispoint and including the mirror segment 380, flexure 320 and thesacrificial layer 515, typically phosphorus-doped silicate glass (PSG).As compared with the structure shown at the last step 437 of FIG. 4B-1,the structure shown in FIG. 5 has been inverted in preparation forbonding to the electronics chip. In the first embodiment, the flexure320 is a two-layer structure with a first flexure layer 330 ofphosphorus-doped polysilicon, and a second flexure layer 340 of SixNy.Although not required in all embodiments, the MEMS device in the firstembodiment includes a temporary handle wafer (not shown in FIG. 5),typically 300 to 500 micrometers thick, used to support the MEMSstructure prior to release in a manner known in the art.

[0054] Continuing again with reference to process steps 445 onwards,shown in FIGS. 4A and 4B, drive circuitry in the form of an integratedcircuit is now introduced. This integrated circuit is the substrate 300on which the flexures and mirror segments will be mounted. The substrate300 is typically fabricated through separate processing in aconventional manner, for example using silicon CMOS techniques not shownhere, and well known in the art. As shown at step 445 in FIG. 4, thesubstrate 300 is typically coated with a passivation layer to protect itfrom the MEMS release agent, which may for example be hydrofluoric acid.As shown in step 447 of FIG. 4, the passivation layer is patterned andetched to expose bond sites on the substrate 300 that are electricallyconnected to a ground plane or to underlying circuits. An electricallyconductive bonding agent 610 is then deposited on these bond sites. FIG.6 is a schematic cross-section through the substrate at the end of step447, showing the locations of the control electrodes 370, the bondingagent 610 and the wiring layer 625.

[0055] Continuing to refer to FIG. 4, at step 450 the MEMS structure500, constructed as described above, is disposed over the substrate 300and the two are then bonded together. At this point the MEMS structure500 still includes the sacrificial layer 515.

[0056] At step 455 the handle wafer of the BSOI wafer is etched awayfrom the MEMS mirror segment, after which the sacrificial layer isreleased from the MEMS structure as shown at step 460. The ICpassivation layer is removed at step 465, typically using an O2 plasmaor appropriate solvent. Finally, an optical coating is deposited on thetop surface of the mirror segments, for example using a shadow-maskedmetal evaporation, in step 470. The resulting device is a completed,integrated DM. FIG. 7A shows a cross-section through a single mirrorsegment and underlying structures, after removal of the handle wafer atstep 455. FIG. 7B shows a cross section through the same structure atthe end of the fabrication process, after the MEMS sacrificial layer 515and circuitry passivation layer have been removed. The device includesthe following elements: IC portion or substrate 300 and MEMS structure500; on the IC portion are shown a control electrode 370, the bondingagent 610 and a wiring layer 625. On the MEMS portion 500 are shown themirror segment 380 and flexure 320, comprising the first flexure layer330 and second flexure layer 340.

[0057] One important aspect of the present invention is theabove-described passivation layer. In the first embodiment of theinvention, an electrically-conductive contact must be establishedthrough the passivation layer at the points where the MEMS structure 500is bonded to the substrate 300. The bonding process can be any suitableprocess that results in a conductive bond, for example gold to goldbonding. To allow the bond material to be deposited onto the ICsubstrate 300, the passivation layer is preferably patternable. In anexemplary arrangement, the passivation layer is completely removableafter the MEMS structure is released in a manner that will not damagethe MEMS structure. This passivation material may be a protectivepolymer material such as a polyimide or parylene.

[0058] Alternatively, the passivation material can be conductive so thatupon removal from the exposed surfaces, electrical contact between theICs and MEMS element is maintained. The passivation material need not bepatterned before bonding as it is selectively removed, where not bondedto the MEMS structures, in the passivation layer removal process. Aconductive polymer or epoxy can be used, for example, EPO-TEK OH108-1 orother similar conductive epoxy made by Epoxy Technology, Inc., ofBillerca, Mass.

[0059] The present invention differs significantly from the prior art inthat it relies on the influence of IRS (as opposed to CTE) in theflexures to elevate the mirror segments above the substrate plane, to amuch greater degree than has been found in the prior art. The“Coefficient of thermal expansion” (“CTE”) describes the linear changein size of a material as a function of temperature, while “Intrinsicresidual stress” (IRS) describes the stress in a material, which isdependent on the grain morphology and crystalline defects of a material.This means that the elevation of the segments above the substrate can befar less sensitive to changes in temperature than for comparable priorart devices. The deflection at the elevated end of each flexure isessentially proportional to the curvature of the flexure, which may bewritten as the sum of two components; a first component proportional tothe intrinsic residual stress in the flexure and a second componentproportional to the CTE mismatches in the flexure. In the firstembodiment of the invention, the first flexure layer is composed ofpolysilicon and the second flexure layer is composed of silicon nitride.This provides a flexure for which the IRS component is larger than theCTE component by a factor of approximately one thousand at normaloperating temperatures, for example in the range 0-100 degrees Celcius.

[0060] Many alternative embodiments of the flexure are possible in whichthe second flexure material is one with a CTE similar to that of thefirst flexure material. If that first material is polysilicon, thesecond material can be a ceramic, such as SiC, or silicon nitride(SixNy), or even polysilicon itself, deposited under differentconditions so as to induce a different grain structure and crystaldefect concentration, and thus different IRS. FIG. 8 is a table thatlists the CTE of some example materials.

[0061] In contrast to the prior art usage of nickel, SixNy isadvantageous because it does not contaminate etchers as Ni does. SixNyis also easier to process because it is a standard IC material depositedby LPCVD. The residual stress of SixNy can be controlled by varying theratios of the reactant gasses, deposition pressure, and the depositiontemperature. For example, a layer deposited with a gas flow ratio of 1:3dichlorosilane to ammonia at 125 mTorr and 800 will yield astoichiometric film (Si3N4) with approximately 1 GPa of residual tensilestress, while 4:1 gas ratio at 140 mTorr and 835° C. will yield a filmcomposition near Si3N3 with approximately 280 MPa of residual tensilestress. To achieve the desired radius of curvature of the flexure,different SixNy stoichiometries can be used, the appropriate choice forwhich may be application-specific.

[0062] The first embodiment of the DM comprises a tiled array of mirrorsegments, supported on flexures and elevated approximately 50micrometers above the substrate. As described, the substrate containselectronic circuits used for controlling and sensing the tip, tilt andpiston motion of the segments. The circuits are controlled viaelectrical signals transmitted, for example, through bond pads on thesubstrate and generated, for example, by a microprocessor in a mannerwell known in the art. The control signals typically containinformation, generated by a wavefront reconstructor, about thecombination of tip, tilt and piston motions for each mirror segmentneeded to compensate for the wavefront aberrations at a given time.“Piston movement” is one of three types of movement used to describeactuation of a mirror segment, and describes translation normal to theplane of the DM aperture. “Tilt”, the second type of movement, ismovement about any first axis that is parallel to the plane of the DMaperture. “Tip”, the third type of movement, is movement about anysecond axis (not parallel to the first axis) that is also parallel tothe substrate.

[0063] The circuits embedded in the substrate 300 decode thisinformation and translate it into a corresponding set of voltages thatare applied to the control electrodes disposed under each mirrorsegment. The electrical potential difference and resulting electrostaticforce between each mirror segment and its three control electrodescauses it to move in tip, tilt and piston, and assume a position andorientation determined by the voltages applied to the three electrodes.This ability to independently orient and position each segment allowsspatially complex wavefront aberrations to be corrected by the DM. Insome implementations of the first embodiment, the substrate alsocontains sense electronics that detect the tip, tilt and piston of eachsegment, for example by measuring the capacitance between the segmentand its three control electrodes. Incorporation of sense electronics canimprove the resolution with which the segments can be controlled.Because the attractive force between a segment and its controlelectrodes increases rapidly as the gap between them diminishes, thecontrol voltages must be limited to avoid pulling segments into contactwith the electrodes. Typically, the maximum operation voltage is chosento be the voltage that causes a segment to travel 25% of the elevationproduced by the flexures. Therefore, the flexure elevation of 50micrometers described in the first embodiment results in a useablemirror stroke of approximately 12 micrometers.

[0064] In a second embodiment of the invention, the structure of the DMis identical to the structure of the first embodiment, except that theground pads and control electrodes are formed on the MEMS part ratherthan the CMOS part. The appearance of the completed device isessentially identical to that of the first embodiment, illustrated inFIG. 3A.

[0065] Fabrication of the second embodiment proceeds in a manneridentical to that used for the first embodiment up to step 435 of FIG.4B. A sacrificial layer is then deposited, patterned and etched to openup anchor points where the ground pads 310 will attach to the firstanchor regions 350 of the flexures. A layer of polysilicon is thendeposited, patterned and etched to define the ground pads 310 and thecontrol electrodes 370. A layer of metal is then deposited, patternedand etched so that it coats the surfaces of the ground pads 310 andcontrol electrodes 370, but does not bridge unconnected structures.

[0066] The CMOS portion 300 of the device is fabricated in the same wayas for the first embodiment, but has bond sites in locations thatcorrespond to both the ground pads 310 and the control electrodes 370 ofthe MEMS structure. The ground pad bond sites are electrically connectedto a ground plane or to circuits in the substrate 300, while the controlelectrode bond sites are connected to the appropriate control and sensecircuits within the substrate 300. The MEMS portion and the CMOS portionare bonded together using a film of anisotropic conductive polymer thatconducts only in a direction normal to the plane of the film. In thisembodiment, the anisotropic conductive polymer acts as both a bondingagent and a CMOS passivation layer. After bonding, the MEMS structuresare mechanically released, for example by HF etching, as in the firstembodiment. Because of the anisotropic nature of the polymer, it doesnot need to be removed from the DM and so the passivation layer removalstep is omitted for this embodiment. As for the first embodiment, thefinal step is the deposition of an optical coating on the top surface ofthe mirror segments.

[0067] The method of operation for the second embodiment is identical tothat for the first embodiment.

[0068]FIG. 9 shows the mechanical structure of a DM according to thethird embodiment of the invention, in a partially exploded perspectiveview. For the sake of clarity, FIG. 9 shows only a singlepiston-tip-tilt mirror segment. However, it will be clear to one skilledin the art that multiple such mirrors may be fabricated side-by-side ona single substrate to form a segmented DM, as was described for thefirst embodiment.

[0069] The third embodiment of the DM comprises a substrate 900, whichmay be a silicon wafer. On top of the substrate 900 are formed a numberof control electrodes 960 that are electrically isolated from oneanother and electrically connected to conductive traces (not shown inFIG. 9) that may either be embedded in the substrate 900 or attached tothe surface of the substrate 900. These traces electrically connect thecontrol electrodes 960 directly to bond pads (not shown in FIG. 9) thatmay be disposed around the perimeter of the DM chip. The controlelectrodes 960 are arranged in groups of three and are rhombic in shape,so that the footprint of each group is essentially hexagonal.

[0070] Disposed around each group of three control electrodes 960, arethree conductive ground pads 910, fabricated from the same material asthe control electrodes 960. The ground pads 910 are electricallyisolated from the control electrodes 960 and electrically connected to aground plane embedded in the substrate 900. Attached to one end of eachground pad 910 is a first anchor portion 950 of a flexure 920. Theflexure, in the third embodiment comprises two layers, a first flexurelayer 930 formed from conductive polycrystalline silicon and a secondflexure layer 940 formed from silicon nitride. The first anchor portion950 is both mechanically and electrically connected to the ground pad910 so that the conductive first flexure layer 930 is held at the samepotential as the ground pad 910. The second flexure layer 940 is rigidlyattached to the top side of the first flexure layer 930 and extends overa portion of the length of the flexure 920. The purpose of the secondflexure layer is to provide a residual stress difference between the topand bottom portions of the flexure 920, causing the flexure 920 to bendup out of the plane of the substrate 300.

[0071] The end of the flexure 920 opposite the first anchor portion 950is electrically and mechanically connected to a hexagonal platform 980.A platform bond site 990, fabricated from a metal, is electrically andmechanically connected to the platform. This platform bond site matchesup with a corresponding segment bond site, also fabricated from a metal,on the underside of a mirror segment 970. The segment bond site is notvisible in FIG. 9, since it is on the underside of the mirror segment970. In the fully assembled DM, the mirror segment 970 is mechanicallyand electrically connected to the platform 980 via these bond sites.Thus the mirror segment 970 is held at some elevation above thesubstrate 900. In the first embodiment, this elevation is on the orderof 50 micrometers. The mirror segment is electrically conductive andtherefore is held at the same potential as the ground pad 910. In thethird embodiment, the mirror segment 970 is hexagonal in shape and isformed from a 20 micrometer-thick layer of single crystal silicon and iscoated on its top surface with an optical coating, which may be a highlyreflective metal layer. The mirror segment diameter in the thirdembodiment is on the order of 500 micrometers.

[0072] In the third embodiment, the DM does not incorporate drive andsense electronics, but does incorporate the improved bimorph flexure.The actuator substrate 900 is fabricated in a method similar to thatused to fabricate the MEMS portion of the first embodiment, but wherethe starting material is a standard silicon wafer rather than a bondedSOI wafer. The ground pads 910, control electrodes 960, electricaltraces and bond pads are defined in a first undoped polysilicon layer,deposited on an insulating silicon nitride layer. Alternatively, thetraces could be fabricated in a buried layer beneath the electrodes thatis electrically isolated in all regions except areas that contact theelectrical traces to electrodes and bond pads. A phosphorous-dopedsilicate glass (PSG) sacrificial layer is then deposited, patterned andetched to open up regions where the first anchor portion 950 of theflexures will connect to the ground pads 910. A second undoped amorphouspolysilicon layer is then deposited followed by a PSG layer. The waferis annealed at 950° C. for six hours to dope and tune the residualstress of the second polysilicon layer to approximately −40 MPa. In thisstep, the sacrificial PSG layer also dopes the first layer ofpolysilicon. The top PSG layer is then removed using a wet HF acid etchand the second polysilicon layer is patterned and etched to define thefirst flexure layers 930 and platforms 980. A layer of silicon nitrideis then deposited, patterned and etched to define the second flexurelayers 940, after which a low-temperature oxide (LTO) is deposited byLPCVD to protect the structures from a later etch. The LTO layer isetched and a metal layer is selectively deposited, for example byelectroplating, to form the bond sites 990 and bond pads disposed aroundthe perimeter of the DM chip.

[0073] The mirror segments 970 are formed on a separate wafer, typicallya BSOI wafer with a 20 micrometer thick device layer. The mirrorsegments are defined using deep reactive ion etching, followed bydeposition of a sacrificial layer (typically PSG) that refills thetrenches between the segments. The sacrificial layer is then patternedand etched to clear access holes for bond sites that match thosedeposited on the actuator substrate 900. A metal layer is thenselectively deposited, for example by evaporation and lift-off, to formthe segment bond sites that will be joined to the corresponding platformbond sites 990.

[0074] The actuators and mirror segments are then assembled and bondedtogether, for example using gold to gold bonding. The mirror-segmenthandle wafer is then removed in a manner known to those skilled in theart, and the sacrificial layers are removed, for example by HF etching,to allow the flexures to lift the mirror segments 970 above thesubstrate 900. Finally, an optical coating is deposited on the topsurface of the mirror segments.

[0075] The third embodiment is operated in a manner similar to the firstembodiment, with the exception that the control voltages used to set theorientation and piston of the mirror segments are generated by driverelectronics on a chip or board that is physically separate from the DMchip. The control electrodes for each mirror segment are connected tothe outputs of the drive electronics for example via bond wireselectrically connected to the bond pads disposed around the edges of theDM chip.

[0076] Accordingly, the invention provides improved methods andstructures for elevating a number of platforms above a substrate and forcontrolling the piston, tip and tilt motions of those platforms. Theresulting structures feature low temperature dependence, small size andpower consumption and high control precision. The methods and structuresmay be used to construct an improved deformable mirror (DM) thatfeatures low temperature dependence, high fill-factor, high controlresolution and large stroke, and which can be fabricated in a smallform-factor at low cost. The ability to integrate drive and senseelectronics on the same chip as the mirror segments allows DMs withlarge numbers of actuators to be realized. The structures and methodsfor producing temperature-insensitive elevated mirror segments and thestructures and methods for assembling the mirror segments on to controland sense electronics can be applied separately or in combination.

[0077] Although the description above contains many specificities, theseshould not be construed as limiting the scope of the invention but asmerely providing illustrations of some of the possible embodiments ofthis invention. For example, the mirror segments can have other shapes,such as square, rectangular, triangular etc.; the mirror segments can besupported by different numbers of flexures; the flexures can beconstructed from any number of materials and comprise any number oflayers, provided their curvature is predominantly caused by IRS, ratherthan CTE differences; the tip, tilt and piston of the mirror segmentscan be controlled by varying the duty cycle of an AC signal applied tothe control electrodes rather than the magnitude of an applied DCsignal; the thicknesses of the layers that comprise the DM can bevaried; the diameters or widths of features such as the mirror segments,flexures and control electrodes can be varied; the number and placementof the control electrodes under each segment can be changed; theelevation of the mirror segments above the substrate can be altered; theactuators need not be electrostatic but could be, for example,piezoelectric or magnetic; the gaps between mirror segments can bechanged; different reflective coatings including both metallic anddielectric coatings can be deposited on the top surface of the segments;different materials and methods can be used to bond the MEMS portion tothe CMOS portion; different passivation materials can be used to protectthe CMOS circuits during MEMS release; the number of mirror segmentscomprising the DM can be varied, etc.

[0078] While numerous specific details have been set forth in order toprovide a thorough understanding of the present invention, numerousaspects of the present invention may be practiced with only some ofthese details. In addition, certain process operations and relateddetails which are known in the art have not been described in detail inorder not to unnecessarily obscure the present invention.

[0079] Thus the scope of the invention should be determined by theappended claims and their legal equivalents, rather than by the examplesgiven.

[0080] The foregoing detailed description of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching. The described embodiments were chosen in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

We claim:
 1. An apparatus comprising: a substrate; and a platformelevated above the substrate and supported by curved flexures, whereinthe curvature of said flexures results substantially from variations inintrinsic residual stress within said flexures.